A polymer film of typically cellulose ester, polyester, polycarbonate, cyclo-olefin polymer, vinyl polymer or polyimide is used in silver halide photographic materials, retardation films, polarizers and image display devices. Films that are more excellent in point of the surface smoothness and the uniformity can be produced from these polymers, and the polymers are therefore widely employed for optical films.
Of those, cellulose ester films having suitable moisture permeability can be directly stuck to most popular polarizing films formed of polyvinyl alcohol (PVA)/iodine in on-line operation. Accordingly, cellulose acylate, especially cellulose acetate is widely employed as a protective film for polarizers.
On the other hand, when transparent polymer films are applied to optical use, for example, in retardation films, supports for retardation films, protective films for polarizers and liquid crystal display devices, the control of their optical anisotropy is an extremely important element in determining the performance (e.g., visibility) of display devices. With the recent demand for broadening the viewing angle of liquid crystal display devices, improvement of retardation compensation in the devices is desired, for which it is desired to suitably control the in-plane retardation Re (this may be simply referred to as Re) and the thickness-direction retardation Rth (this may be simply referred to as Rth) of the retardation film to be disposed between a polarizing film and a liquid crystal cell. In particular, since transparent polymer films having a negative Rth are not easy to produce, and it is desired to produce them in a simplified manner.
For producing transparent polymer films having a negative Rth, disclosed is a method of forming a vertically-aligned liquid crystal layer on an isotropic transparent polymer film serving as a support (e.g., see JP-A-6-331826), but the method is problematic in that the production process according to it is complicated and that the producibility is lowered owing to the retardation fluctuation caused by the non-uniformity of the alignment and the film thickness.
For producing transparent polymer films having a negative Rth, also disclosed is a continuous production method comprising sticking a thermal-shrinking film to a transparent polymer film, then thermally stretching it and thereafter peeling the thermal-shrinking film (e.g., see JP-A-5-157911 and JP-A-2000-231016). Examples in these references show that the polycarbonate films produced according to the method have a negative Rth. However, the method is problematic in that it consumes a large quantity of a thermal-shrinking film and that the quality (e.g., retardation value, slow axis direction) of the produced films is not uniform. The problems are especially serious with polymers of high elasticity such as cellulose esters and hydrophilic polymers.
For producing transparent polymer films having a negative Rth, also disclosed is a method of using a cellulose ester film having a high substitution degree of acetyl group (e.g., see JP-A-2005-120352). According to the method, films having suitable moisture permeability can be obtained, but the method is problematic in that the energy necessary for polymer dissolution is large and that the produced films could not have a sufficient Re.